1. Field of the Invention
This invention relates generally to a system and method for estimating a state-of-charge (SOC) for a rechargeable energy storage system (RESS) and, more particularly, to a system and method for accurately estimating the battery SOC during plug-in charge mode.
2. Discussion of the Related Art
Electric-only and hybrid vehicles, such as battery electric vehicles (BEVs), range extended electric vehicles (REEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs) and fuel cell hybrid electric vehicles (FCHEVs), are becoming increasingly more prevalent. Hybrid vehicles generally combine a rechargeable energy storage system (RESS) with a main power source, such as internal combustible engine or fuel cell system. In one instance, a RESS may be a high voltage battery having a number of battery cells. These cells can be of different chemistries including lithium-ion, lithium-iron, nickel metal hydride, lead acid, etc. A typical high voltage RESS for a BEV, REEV, HEV, PHEV, or FCHEV may include 196 cells providing about 400 volts. Further, the RESS may include individual modules where each module is constructed or made up of a number of interconnected cells. The individual cells may be electrically coupled in series, or a series of cells may be electrically coupled in parallel, where a number of cells in the module are connected in series and each module is electrically coupled to the other modules in parallel. Different vehicle designs include different RESS configurations that employ various trade-offs and advantages for a particular application.
The effectiveness of RESS control and power management is essential to vehicle performance, fuel economy, RESS life and passenger comfort. For RESS control and power management, two states of the RESS, namely, state-of-charge (SOC) and power, need to be predicted, estimated, and monitored in real time because they are not easily measurable during vehicle operation. SOC is a term that refers to the stored charge available to do work relative to that which is available after the RESS has been fully charged. SOC can be viewed as a thermodynamic quantity, enabling one to assess the potential energy of the system. The SOC of the RESS in a vehicle system, such as a BEV, REEV, HEV, PHEV or FCHEV, is important with respect to vehicle efficiency, emissions and power availability. For example, a vehicle operator or an onboard controller might utilize the SOC for the purpose of regulating the operation of the RESS.
One way to estimate SOC is during a plug-in charge mode, where the vehicle is restored to full charge by connecting a plug to an external electrical power source outside the vehicle. However, existing technologies for estimating SOC generally employ non-adaptive IR compensation and require signal excitation to estimate dynamic impedance. For example, it is known to use a current only based SOC estimation technique such as coulomb counting, which depends on a known starting point for an open circuit voltage and initial SOC from the last driving cycle to be correct. The coulomb counting technique can only be used if the starting points are known with absolute certainty, which is difficult because one cannot predict how long the vehicle has been off, driver behavior or how much polarization has occurred in the cells.
Another method for estimation is to employ voltage based SOC calculations such as regression in the form of a recursive least square algorithm. However, these methods require sufficient excitation in the voltage and current signals. During a plug in charge event utilizing constant current, there is too little excitation to allow for algorithms dependant on excitation to calculate SOC with any fidelity. As a result, the voltage-based SOC is not valid most of the time during a plug-in charge.
Therefore, what is desired is a system and method configured to provide accurate and stable information of SOC during a plug-in charge mode to maintain precise charge control to avoid overcharge or undercharge of the RESS when adaptive regression is unable to provide an accurate and stable SOC due to lack of excitation.